Merge branch 'mh/doc-remote-helpers'

* mh/doc-remote-helpers:
  git-remote-helpers.txt: clarify options & ref list attributes
  git-remote-helpers.txt: clarify command <-> capability correspondences
  git-remote-helpers.txt: rearrange description of capabilities
  git-remote-helpers.txt: minor grammar fix
  git-remote-helpers.txt: document missing capabilities
  git-remote-helpers.txt: document invocation before input format